Environmental factors such as stress and diet could be affecting the genes of future generations leading to increased rates of obesity, heart disease and diabetes.

A study of people suffering post-traumatic stress disorder (PTSD) after the 9/11 attacks in New York made a striking discovery.

The patients included mothers who were pregnant on 9/11 and found altered levels of the stress hormone cortisol in the blood of their babies.

This effect was most pronounced for mothers who were in the third trimester of pregnancy suggesting events in the womb might be responsible.

We are finding a fundamental part of human biology that we can't afford to ignore

Professor Mark HansonSouthampton University

The changes are thought to occur through the workings of a set of instructions that sit on top of our DNA; chemical marks which determine whether a gene is switched on and active, or remains silent.

This is epigenetics, an additional layer of information which scientists are now beginning to understand.

External influences

As an embryo develops, it experiences wave upon wave of epigenetic changes.

This was thought to be a fixed process, but researchers now think it could be subject to external influences, enabling a developing animal to adapt to its environment.

Mum is sending a signal to junior - things are tough out here

Professor Jonathan SecklEdinburgh University

"The foetus and newborn get information, primarily from the mother, about the world that it will grow up into," said Professor Peter Gluckman, from the University of Auckland.

But what happens if that information is not quite right?

Professor Gluckman is one of a number of scientists who believe that increased rates of obesity, heart disease and diabetes have their roots early in development; that a developing foetus' experience of factors such as nutrition or stress could alter its susceptibility to disease as an adult.

Professor Jonathan Seckl from Edinburgh University has shown that exposure to abnormally high levels of stress hormones in the womb can alter an animal's biology.

"Imagine this is an animal being born into a harsh environment, or a child being born into a war zone he explains.

"Mum's sending a signal to junior - things are tough out here - so you had better set up your physiology, your metabolism, your behaviour, in order to expect trauma."

A beneficial adaptation in the short term, but over a lifespan, these physiological changes bring an increased risk of disease.

And there is increasing evidence it could affect more than one generation.

Altered levels of the stress hormone cortisol have been found in another group of PTSD patients - children of Holocaust survivors, born not months, but years after their parents were exposed to traumatic stress.

"They weren't a foetus, but they might have been an egg," said Professor Seckl.

So influences in the environment may affect not just a child in the womb, but the instruction manuals of the egg and sperm cells which become the next generation.

Food shortage impact

Other transgenerational effects have been observed.

In northern Sweden, researchers found that food shortages experienced by grandfathers as children had an effect on the longevity of their grandsons.

And in the Avon Longitudinal Study of Parents and Children in Bristol, fathers who took up smoking before puberty had sons who were more likely to be obese.

If correct, this research could have huge implications for public health.

"We really have to focus attention on the diet, lifestyle and wellbeing of young people, especially young women of reproductive age," says Professor Mark Hanson, from Southampton University.

"We are finding a fundamental part of human biology that we can't afford to ignore."

Radio 4's The Switching Point is broadcast on Thursday 20 & 27 December at 2100GMT.